hint using nogncheck on disallowed includes

Change-Id: Ibdea79d52f110b89aac7b441f509509d7a8124bf
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/17440
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Takuto Ikuta <tikuta@google.com>

src/gn/header_checker.cc

diff --git a/src/gn/header_checker.cc b/src/gn/header_checker.cc
index 197084b..0abf71a 100644
--- a/src/gn/header_checker.cc
+++ b/src/gn/header_checker.cc
@@ -653,7 +653,9 @@
           targets_with_matching_toolchains.size() >
       1)
     msg += "at least one of ";
-  msg += "which should somehow be reachable.";
+  msg += "which should somehow be reachable.\n";
+  msg += "This might be a false alarm if you are using conditional include. "
+         "See \"gn help nogncheck\".";
 
   // Danger: must call CreatePersistentRange to put in Err.
   return Err(CreatePersistentRange(source_file, range), "Include not allowed.",